Conventionally, automatic transmissions mounted on a vehicle and the like uses, for example, a torque converter as a fluid transmission device to smoothly execute a shift of change of operation states such as start, stop and the like. The torque converter includes, for example, a fluid transmission mechanism, a lock-up clutch mechanism, and a damper mechanism. Then, the torque converter transmits power, which is transmitted from a power source (drive source) to a front cover when the lock-up clutch is turned OFF, to an output shaft (for example, an input shaft of a transmission) via a working oil as a working fluid in a fluid transmission mechanism, whereas the torque converter directly transmits a drive force, which is transmitted to the front cover when the lock-up clutch is turned ON, to the output shaft via an engagement member in the lock-up clutch mechanism without via the working fluid in the fluid transmission mechanism. At the time, the damper mechanism damps a vibration when the drive force is transmitted.
As the conventional fluid transmission device, a vehicle power transmission device described in, for example, Patent Literature 1 starts to engage a lock-up clutch at a time before an output of a turbo charger is increased and a boost thereof is increased by starting to engage the lock-up clutch when a speed ratio, which is a ratio of a turbine revolution number to a pump revolution number, is 0.7 or less in a vehicle power transmission device composed of a combination of a turbo engine and a fluid coupling provided with a lock-up clutch. With the operation, since the vehicle power transmission device can complete to engage the lock-up clutch before an engine revolution number and output torque are increased by the increase of the boost, the vehicle power transmission device can make a timing at which the lock-up clutch is engaged after a vehicle starts appropriate.